The invention relates to a burner operated with liquid fuel for heating devices, especially vehicle heaters, with a low-pressure atomizer arranged at one end of a combustion chamber, this atomizer being formed by a burner insert and a nozzle, the latter projecting into the end of the burner insert facing away from the combustion chamber, this nozzle being fed with fuel and a primary air stream.
In burners of this type, combustion residues tend to accumulate in the zone of the burner insert. Such deposits require a relatively frequent servicing of the burner. Although the depositing of combustion residues can be basically counteracted by increasing the amount of primary air for the burner, such a step results in impaired cold-start properties in conventional burners.
Thus, the invention has for an object, providing a burner that permits a prolongation of the servicing intervals by reduced accumulation of residues without a concomitant impairment of the cold-start properties.
Starting with a burner of the type described above, this object has been attained according to a preferred embodiment of the invention by providing that at least the primary air quantity passing through the burner is automatically controllable in dependence on the burner temperature. In a burner constructed according to this invention, the amount of primary air can be set so that a perfect cold start is ensured by a rich mixture. As soon as the burner has reached its operating temperature, however, an automatic increase in leanness of the mixture is effected by increasing the amount of primary air fed to the burner. Thereby, deposits of combustion residues are effectively prevented.
According to a preferred embodiment of the invention, the burner insert is equipped with a feed line for supplemental primary air stream. An air valve controlled in dependence on the burner temperature is located in this feed line, by means of which the feed of supplemental primary air can be blocked at least partially when the burner is cold. In the starting phase, the operation is thus conducted without supplemental primary air, or at least with only a small quantity of such air. However, once the burner has been brought to the operating temperature and atomization or vaporization of the liquid fuel is ensured, the feed line for the supplemental air stream is fully opened. The total amount of primary air is correspondingly increased.
The feed line for supplemental primary air can suitably be constituted by a ring chamber located in the zone of the end of the nozzle on the side of the combustion chamber. Preferably, a bimetal ring is disposed in this ring chamber, which ring release or at least partially blocks the passage of supplemental primary air in dependence on the temperature. Such a solution is distinguished by a particularly simple structure and high operating safety.
Advantageously, the ring chamber is equipped with one or several supplemental primary air inlet ports, forming an acute angle with the radial direction. This effects a swirl in the introduced supplemental primary air. The supplemental primary air flows in contact with the wall of the burner insert and effectively blasts away any fuel droplets or any originating combustion residues.
The burner insert is suitably provided with a nozzle connection (socket) and a burner tube lying coaxially thereto, both of which define the ring chamber and are in communication with each other, preferably via a heat reflux throttle (choke). The heat reflux throttle limits the heat returned to the nozzle connection to an amount sufficient, on the one hand, for fuel vaporization, but, on the other hand, to avoid a heating up of the nozzle connection to a temperature at which the burner fuel will undergo a cracking process.
The burner tube may simply be equipped with a thin-walled tubular extension on the end facing away from the combustion chamber, in order to form the heat reflux throttle; the end of the nozzle connection on the combustion chamber side extends into this extension to such a degree that its front face lies at a spacing from the adjacent end face of the burner tube.
The bimetal ring is advantageously mounted with its outer periphery to the end of the nozzle connection on the combustion chamber side and can be deflected under the effect of the operating temperature of the burner into the space between the forward end face of the nozzle connection and the ajacent end face of the burner tube.
To exclude a damming-up effect upstream of the burner tube, the cross section of the burner tube inlet port is preferably larger than the cross section of the primary air/fuel outlet port of the nozzle connection.
In a further development of the invention, the burner insert is additionally provided with a constriction ring axially at a spacing from the end of the nozzle on the combustion chamber side, whereby a heat accumulation effect is ensured in the zone of the burner tube lying in front of the constriction ring, and as a result an especially quiet combustion can be obtained.
The burner insert can be equipped with secondary air inlet ports in a zone located axially at a spacing from the nozzle, these inlet ports being suitably fashioned as swirl bores. Advantageously, the burner insert is seated in a burner head defining, together with the outer surface of the burner insert, an air chamber. From this air chamber, the burner can be fed with supplemental primary air as well as with secondary air.
The residue deposits can be even further reduced in another embodiment of the invention by providing that, additionally, the amount of a secondary air stream fed to the burner is likewise automatically controllable in dependence on the burner temperature. In this arrangement, an air valve controlled in dependence on the burner temperature is preferably disposed in a combined feed line for the primary and secondary air streams; by means of this air valve, the feed of primary and secondary air can be at least partially blocked off when the burner is cold. In the start-up phase, the valve reduces the amount of combustion air introduced in total to any desired degree and then opens up the combined feed line for the primary and secondary air streams upon attainment of the operating temperatures, thus correspondingly increasing the quantity of combustion air.
The combined feed line for primary and secondary air can suitably constitute a ring chamber located in the region of the end of the nozzle on the combustion chamber side; a combustion air connection pipe terminates in this ring chamber, preferably in a substantially tangential direction. This arrangement ensures an extensively uniform and effective distribution of the combustion air. To provide automatic control of the primary and secondary air qualities, a bimetallic strip is advantageously located at the entrance point of the combustion air connection pipe into the ring chamber, this strip releasing or at least partially blocking the air feed in dependence on the temperature. Such a solution is distinguished by particular constructional simplicity and high operating safety. In this connection, the burner insert suitably exhibits a nozzle connection in communication with the ring chamber via primary air inlet ducts and secondary air inlet ducts being in communication with a space lying in the zone of the forward end section of the nozzle connection.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.